1. Field of the Invention
The present invention relates to radio frequency (RF) transponders and radio frequency identification (RFID) systems, and more particularly, to an identification protocol that includes at least two identification methodologies.
2. Description of Related Art
In the automatic data identification industry, the use of RF transponders (also known as RF tags) has grown in prominence as a way to track data regarding an object on which an RF transponder is affixed. An RF transponder generally includes a semiconductor memory in which information may be stored. An RF base station containing a transmitter-receiver unit is used to query an RF transponder that may be at a distance from the base station. The RF transponder detects the interrogating signal and transmits a response signal containing encoded data back to the base station. RF and RFID systems are used in applications such as inventory management, security access, personnel identification, factory automation, automotive toll debiting, and vehicle identification, to name just a few.
Such RFID systems provide certain advantages over conventional optical indicia recognition systems (e.g., bar code symbols). For example, the RF transponders may have a memory capacity of several kilobytes or more, which is substantially greater than the maximum amount of data that may be contained in a conventional one-dimensional bar code symbol. The RF transponder memory may be re-written with new or additional data, which would not be possible with a printed bar code symbol. Moreover, RF transponders may be readable at a distance without requiring a direct line-of-sight view by the interrogator, unlike bar code symbols that must be within a direct line-of-sight and which may be entirely unreadable if the symbol is obscured or damaged.
RF transponders may either be “read-only” (R), in which data can only be read from the RF transponder, or “read/write” (R/W), in which data can both be read from and written to the RF transponder. The traditional method of writing (and reading) data to (and from) a transponder is to first interrogate the transponder to determine its ID number. The ID number is then used by the RFID base station to construct a proper request (or command). This is because a Write command, for example, typically includes (i) a write opcode, (ii) an address of a memory device, (iii) data to be stored at that address, and (iv) the ID number of the RF transponder at issue. The RF transponder's ID number is an important component of the command in that it allows the RF transponder to determine which transponder the request is directed toward. In other words, if the ID number included in the write request does not match the ID number stored on the RF transponder, the request may be ignored by the transponder.
Identifying an RF transponder (or its ID number) is more difficult when multiple RF transponders are within the RFID base station's communication range (or RF field). This is because the identification request sent by the base station is typically a general request (i.e., is not directed toward any specific transponder) and can therefore result in a plurality of ID numbers being transmitting simultaneously. Such a transmission typically results in the base station receiving ID numbers (or signals) that are unintelligible.
One method of dealing with this situation is to use a random number generator (RNG) and a counter to identify at least two groups of transponders—e.g., ones that are to transmit their ID numbers and ones that are not. Specifically, each transponder includes a counter that is originally set to zero, an RNG that produces either a one or a zero, and instructions to (i) transmit its ID number if its counter is zero, (ii) run its RNG in response to a “Fail” command and to increment its counter by one if the RNG result is zero, and (iii) decrement its counter by one in response to a “Success” command.
Assume, for example, that two transponders having counters equal to zero enter a base station's RF field. The transponders would respond by transmitting their ID numbers. This is because the transponders are adapted to transmit their ID numbers if their counters are equal to zero. This results, however, in the base station receiving unintelligible ID numbers (as previously discussed). The base station would then respond by transmitting a Fail command.
A Fail command prompts each transponder to perform three operations: (1) run its RNG, (2) increment its counter by one if the RNG result is zero, and (3) transmit its ID number if the RNG result is one. Given the RNG's 50/50 odds of producing a one or zero, the Fail command will likely result in one transponder (e.g., a first transponder) incrementing its counter by one and the other transponder (e.g., a second transponder) leaving its counter at zero and transmitting its ID number.
The base station may then use the ID number to place the first transponder in a “data exchange” (or mute) state. This is typically done by transmitting a Read command, which includes a particular ID number (e.g., the ID number of the transponder at issue) and a start address of the data to be read. Because the start address of an ID number on a transponder is typically zero, a zero address is commonly used in the Read command. This allows the ID number to be confirmed or acknowledged three different times. Specifically, (i) the ID number is originally transmitted by the transponder, (ii) the transponder accepts a read command that includes the ID number, and (iii) the transponder responds to the read command by transmitting its ID number (i.e., the requested data). From this point on, the first transponder will remain mute until it receives a specific command (e.g., a command that includes the first transponder's ID number).
The base station will then transmit a Success command, prompting the second transponder to decrement its counter by one. Because the second transponder's counter is now zero, it responds by transmitting its ID number. The identification and isolation process continues as previously described. Such a system and method is discussed in greater detail in U.S. Pat. No. 5,550,547, which is incorporated herein, in its entirety, by reference.
While such a method is advantageous in identifying and isolating individual RF transponders, its structure is somewhat regimented and does not provide (in and of itself) flexibility in the area of robustness and efficiency. For example, there may be situations where it would be advantageous to identify individual transponders in less time than it would take to perform the aforementioned process, even if the identification is less precise. Accordingly, it would be desirable to provide an identification protocol that utilizes multiple identifying methodologies, allowing tradeoffs between robustness and efficiency.